Method and apparatus for increasing the efficiency of corona charging



June 30, 1970 c. F. GALLO ET AL 3,517,995

METHOD AND APPARATUS FOR INCREASING THE EFFICIENCY OF CORONA CHARGING Filed on. 2, 1967 PIIIIIIIIIIIIII INVENTORS.

CHARLES F. GALLO LGIRD LEIG ATTORNEYS United States Patent 3,517,995 METHOD AND APPARATUS FOR INCREASING THE EFFICIENCY OF CORONA CHARGING Charles F. Gallo, Penfield, and Algird G. Leiga, Pittsford,

N.Y., assignors to Xerox Corporation, Rochester, N.Y.,

a corporation of New York Filed Oct. 2, 1967, Ser. No. 672,336 Int. Cl. G03g 13/00 US. Cl. 355-17 6 Claims ABSTRACT OF THE DISCLOSURE The deleterious effect of optical radiation from a corona charging unit is minimized by the use of a special photoconductor comprising a photoconductive layer which has been doped so as to render it insensitive tooptical radiation from one side but which nevertheless remains sensitive to imagewise radiation from the other side through a transparent substrate supporting the photoconductive material.

BACKGROUND OF THE INVENTION In general the present invention relates to corona charging and more specifically to a method of increasing the efficiency of corona charging. Corona charging has become almost universally used as a method sensitizing xerographic photoreceptors. This technique has gained the widest commercial acceptance and is described in numerous US. patents, for example, US. Pat. Nos. 2,588,699, 2,777,957, 2,836,725, 2,885,556, 2,922,883. Essentially, this corona technique consists of spacing a filament or plurality of filaments slightly from the surface of a xerographic plate which has its conductive base grounded and applying a high potential to the filament so that an electrical breakdown, that is, corona discharge, occurs between the filament and the plate, thus, serving to raise its level of electrostatic charge with respect to ground potential. This increase in potential is achieved by a large flux of ions which are created by the corona discharge of the wire or wire array which is maintained at a high potential and supportednear the photoconductor. The ions produced by the corona wire result in a deposition of charge on the photoreceptor and sensitize its surface for a later exposure or the like in a xerographic process. I

It has been discovered that the electromagnetic radiation emitted from the charging corona can substantially reduce the charging efficiency of deposited charges, The

deterioration in the charging efficiency may be in excess of 10% for specific photoreceptor combinations. The importance of this problem increases as the sensitivity of the photoconductors increases for use in higher operational speed configurations.

The prior art methods and devices for corona charging produce less than the desired exciency in corona charging and considerable difficulty is encountered as the photoreceptor being charged becomes more sensitive. This difficulty arises from the fact that the electromagnetic radiation produced by the charging corona serves to decrease the charging efficiency of that corona.

SUMMARY OF THE INVENTION Accordingly, it is an object of this invention to pro vide a new highly unobvious and effective method which will overcome the deficiency of the prior art as described above.

It is a further object of this invention to provide a corona charging method which will increase the efiiciency of corona charging.

Another object of .this invention is to provide a method a transparent substrate allows Ice of corona charging which will result in the higher efficiency for high speed, sensitive photoconductors.

Other objects and a fuller understanding of the invention may be had by referring to the following description and claims taken in conjunction with the accomanpanying drawings.

The present invention overcomes the deficiencies of the prior art and achieves it sobjectives by utilizing special photoconductors which have a doped surface or in which the entire volume of the photoconductor has been doped. The doping renders the photoconductors effectively insensitive to optical radiation from one side while imagewise exposure through the substrate from the opposite or preferred direction. Thus, the radiation produced by the charging unit has no or at most a minimal effect upon the photoconductor as it strikes the photoconductor from one direction which for simplicity we may refer to as the doped side. However, full sensitivity of the photoconductor is obtained by exposure through its opposite or backside, that is, through the transparent substrate.

BRIEF DESCRIPTION OF THE DRAWINGS In order to facilitate the understanding of this invention reference will now be made to the appended drawings of the preferred embodiments of the present invention. The drawings should not be construed as limiting the invention but are exemplary only. In the drawings:

The figure is a cross-sectional representation of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT A preferred embodiment of the present invention is shown in the attached figure in which a corona charging unit 10 consisting of corona wire 12 and electrostatic shield 14 is provided in close proximity to the photoreceptor to be charged 16. The photoreceptor to be charged is a special photoreceptor consisting of a transparent substrate or plate electrode 18 which may be constructed of NESA glass or other suitable electrically conductive backing electrode material which is transparent. The normal state of the backing electrode 18 is a grounded state but other polarities with respect to the corona charging unit may be utilized so long as a potential difference between plate electrode 18 and corona wire 12 exists sufficient to draw charges in the ionization process from corona wire 12 toward plate electrode 18. Transparent substrate 18 is covered by a layer of photoconductive material 20 which has been doped with special materials 22 which render the upper surface of the photoconductive layer 20 insensitive to optical, electromagnetic radiation produced by the corona charging wire 12. The doping material 22 may be applied as a layer such as shown in the figure or may be applied in bulk throughout the photoconductive layer. Depending upon the materials employed the doping material either as a layer or in bulk will result in a preferred initial effect upon hole-electron mobility so as to effectively render the photoconductor insensitive to the electromagnetic radiation produced by a charging corona from one side of the photoconductor 20. The photoconductive layer 20 remains, however, sensitive to imagewise radiation from the other side or the backside, that is, through the transparent substrate 18. The imagewise radiation may be produced by shining light from source 24 through a collimating system 26 and thence through a transparent or translucent document 28 and focusing the image of that document by means of a projection lens 30 onto the backside of photoconductive layer 20 through the transparent electrically conductive glass backing electrode 18. It should be noted that an opaque projection system such 3 as shown in Pat. Nos. 2,168,225; 2,811,892; 2,986,062 or 3,166,419 may be utilized for projecting opaque documents or image configurations onto the backside of the photoconductor 20.

Further, it should be noted that the configurations shown in the accompanying figure for the photoconductive layer and its adjacent layers need not be a fiat plane configuration but may take on many other configurations well known in the art. For example, it may have the configuration of a belt or a drum-type photoconductive plate as is well known in the art and shown in Pat. No. 3,045,- 587. Additionally, any optical system may be employed for projecting the image configuration through the transparent layer 1 8 to the photoconductor 20. For example. in the drum configuration an internal light source and an axial optical system may be employed to produce the image radiation pattern as desired on photoconductor 20 in a manner well known in the prior art. As has been noted, opaque projection systems may be employed for producing the image configuration on photoconductive layer 20. The present figure is merely provided to diagrammatically represent the concept of the present invention and one embodiment as a preferred embodiment, however, any other suitable configuration employing the concept of this invention may be employed within the scope of the present invention.

If one desires to achieve positive charging, the photoconductive layer 20 may be composed of selenium and its doped material 22 may be a hole trapping impurity such as thallium to render it insensitive to optical radiation from the topside as produced by corona wire 12.

In the case of negative charging, the selenium layer 20 may be doped with an electron trapping impurity such as chlorine to form layer 22 or in bulk.

'While the thallium doped selenium and the chlorine doped selenium are recited here as preferred materials for the preferred embodiment of the present invention it is clear that many other materials and variations may be employed. For example, any suitable photoconductive insulating layer may be used in carrying out the invention. Typical photoconductive insulating layers include amorphous selenium, alloys of sulfur, arsenic and tellurium with amorphous selenium. Amorphous selenium may be doped with hole trapping materials such as thallium, cadmium sulfide, cadmium selenide, and the like. Particulate photoeonductive materials such as zinc sulfide, zinc cadmium sulfide, French process zinc sulfide, metal-free phthalocyanine, cadmium sulfide, cadmium selenide, zinc cellate, cadmium sulpho-selenide, linear quinacridones, and the like dispersed in insulating inorganic film binder forming binders such as glass or insulating organic film forming a binder such as an epoxy resin, a silicone resin, and alkyd resin, a styrene butadiene resin, a wax or the like may, also, be utilized. Other typical photoconductive insulating materials include blends, copolymers, terpolymers, etc. of photoconductors and non-photoconductive materials which are either copolymer or miscible together to form solid solutions and organic photoconductive materials of this type include anthracene, polyvinyl alanthracene, anthraquinone, oxidiazole derivatives such as 2,5-bis-(p-amino-phenyl-l), 1,3,4-oxidiazole; 2-phenylbenzoxazole; and charge transfer complexes made by complexing resins such as polyvinylcarbazole, phenol-aldehydes, epoxies, phenoxies, polycarbonates, melamines, etc. with Lewis acids such as phthalic anhydride; 2,4,7- trinitrofiuorenone metallic chlorides such as aluminum, zinc or ferric chloride; 4,4-bis (dimethyl-amino) benzophenone; chloram'l picric acid; 1,3,5-trinitrobenzene; 1 chloro-anthraquinone; bromal; 4 nitrobenzaldehyde; 4-nitro phenol, acetic anhydride, maleic anhydride, boron ,trichloride maleic acid, cinnamic acid, benzoic acid, tartaric acid, malonic acid and mixtures thereof.

It should be noted with reference to the preferred embodiment of the present invention, for example, that selenium in its amorphous form doped with a small amount of elemental thallium (for example 0.05% by weight) admirably performs the hole trapping function although any other suitable hole trapping material may be employed.

Other typical hole trapping materials than thallium doped selenium include phosphorus, sulfur, tellurium, barium sulfide, tin, barium carbonate, cadmium sulfide, and cadmium sulpho-selenide. The hole trapping material may be employed throughout the whole thickness of the photoconductor or only as an upper layer on the order of a few microns thick, since either structure will serve the function of trapping holes in the bulk near the upper surface of the photoconductor.

As has been noted above for negative charging the selenium may be doped in the preferred embodiment with an electron trapping impurity such as chlorine or other member of the halogen family and utilized in a similar manner to that described.

Thus, in operation, the top surface of the plate may be charged by means of a conventional corona charging unit which will produce a certain amount of electromagnetic radiation which would have a deleterious effect upon a conventional selenium or other photoconductive plate. Indeed the effect is most pronounced when photoconductors in the group listed above are more sensitive than selenium are employed. The charge is deposited on a doping layer 22 which overcoats the photoconductive layer 20 or on the top surface of the bulk doped photoconductive layer 20. The doping material serves to prevent the deleterious efiects of the optical electromagnetic radiation produced by corona charging wire 12 by performing a hole or electron trapping function depending on whether the potential applied to corona Wire 12 is positive or negative. The image to be produced by conventional xerographic means may be projected through the transparent substrate 18 onto photoconductive layer 20 and the image pattern produced on the doped layer altered by alterations in the photoconductivity layer 20 in a manner similarly analogous to that in conventional xerography and all other steps normally taken in conventional xerography can then be performed in their usual manner as understood by those skilled in the art.

Although a specific preferred embodiment of the present invention has been described in the detailed description above, the description is not intended to limit the invention to the particular forms or embodiments disclosed herein since they are to be recognized as illustrative rather than restrictive. It will be obvious to those skilled in the art that the invention is not so limited. The invention is declared to cover all changes and modifications of the specific example of the invention herein disclosed for purposes of illustration which do not constitute departures from the spirit and scope of the invention.

What is claimed is: 1. A method of increasing the efficiency of corona charging comprising:

'(a) charging a first surface of a doped photoconductive layer, said photoconductive layer being doped with a charge trapping impurity in bulk throughout said layer whereby said photoconductive layer is rendered insensitive to optical radiation impinging thereon, and (b) exposing the opposite surface from said first surface of said photoconductive layer to a pattern of light and shadow whereby an imagewise configuration of charge is formed on said first surface of said photoconductive layer. 2. A method of increasing the efficiency of corona charging comprising:

(a) charging a doped surface of a photoconductive layer, said doped surface comprising a charge trapping impurity whereby said photoconductive layer is rendered insensitive to optical electromagnetic radiation impinging thereon, and

'(b) exposing the other surface of said photoconduce tive layer to a pattern of light and shadow whereby an imagewise configuration of charge is formed on said doped surface of said photoconductive layer.

3. The method of claim 2 wherein said charge trapping impurity comprises an electron trapping impurity and wherein said doped surface of said photoconductive layer is charged negatively.

4. The method of claim 3 wherein said electron trapping impurity is a halogen.

5. The method of claim 2 wherein said charge trapping impurity comprises a hole trapping impurity and wherein said photoconductive layer is charged positively. 6. The method of claim 5 wherein said hole trapping impurity is thallium.

References Cited UNITED STATES PATENTS 3,041,166 6/1962 Bardeen 961.5

RALPH G. NILSON, Primary Examiner 0 A. L. BIRCH, Assistant Examiner 

